Strange Behavior s/ vs c\

"n22e113 [email protected] [sed-users]" <[email protected]> Mon, 14 Dec 2015 21:28:20 -0500

Newsgroups	gmane.editors.sed.user
Message-ID	<[email protected]>

Hi,Seders,
Can someone have a look (Debian 8/GUN sed 4.2.2)?
$ echo 'mail_location=mbox:~/mail:INBOX=/var/mail/%u' |\
  sed -e 's/^mail_location\(.*\)$/#mail_location\1/'
#mail_location=mbox:~/mail:INBOX=/var/mail/%u

$ echo 'mail_location=mbox:~/mail:INBOX=/var/mail/%u' |\
  sed -e '/^mail_location\(.*\)$/ c\#mail_location\1/'
#mail_location1/

Results are not the same with s/ & c\!
Q. What am I do wrong?